What is TypeVarTuple and Unpack for variadic generics?

Q: What is TypeVarTuple and Unpack for variadic generics?

**Target Audience:** Python developers comfortable with `TypeVar` and generics who meet `*Ts` in library stubs, or need to type functions that forward `*args`. **Difficulty:** Hard **Python version:** `TypeVarTuple`, `Unpack` and the `*Ts` star syntax are in `typing` from Python 3.11. Runtime behaviour checked on CPython 3.11.17, 3.12.3 and 3.13.16, and in the page runner (Pyodide 0.25.1, CPython 3.11.3). Type-checker output is from mypy 2.3.1 with `--python-version 3.11`. > **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. All output shown was produced by actually running the code; the **Verified** section quotes it in full. It builds on [generics and TypeVar](/interview-question/what-are-generics-and-typevar-in-python); read that first if `T = TypeVar("T")` is new. ## 1. Why This Even Matters — A Story First A delivery service lets you book a courier for "a parc

The problem

Question presented to candidate: "A normal TypeVar stands for one type. How would you type a function like call_soon(callback, *args) so the checker verifies that args match the callback's parameters, or an Array class whose type includes its shape, like Array[float, Height, Width]? What are TypeVarTuple and Unpack?"

What a strong answer should cover:

TypeVarTuple (PEP 646, Python 3.11) is a placeholder for any number of types, in order; it is always used unpacked, as *Ts or the equivalent Unpack[Ts]
Variadic generic classes: class Array(Generic[DType, *Shape]) lets Array[float, Height, Width] carry a shape; the motivating use is array shapes in NumPy-style libraries
Typed *args: def f(*args: *Ts) -> tuple[*Ts] keeps each argument type; Callable[[*Ts], R] with *args: *Ts ties a callback to its arguments (how typeshed types asyncio call_soon)
Combining with TypeVar for prefix/suffix (tuple[T, *Ts]), and unpacking concrete tuple types (*tuple[int, ...])
Limits: only one TypeVarTuple per parameter list, *Ts only on *args, no keyword arguments (use ParamSpec for that), the runtime checks none of it; 3.12 syntax class Array[DType, *Shape], 3.13 defaults

Code / implementation expected: Optional — a typed *args function or a callback scheduler using Callable[[*Ts], R], with what the checker infers.

typing
typevartuple
generics

Try it yourself

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Solution

Target Audience: Python developers comfortable with TypeVar and generics who meet *Ts in library stubs, or need to type functions that forward *args. Difficulty: Hard Python version: TypeVarTuple, Unpack and the *Ts star syntax are in typing from Python 3.11. Runtime behaviour checked on CPython 3.11.17, 3.12.3 and 3.13.16, and in the page runner (Pyodide 0.25.1, CPython 3.11.3). Type-checker output is from mypy 2.3.1 with --python-version 3.11.

How to read this doc: Every idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact words an interviewer expects. All output shown was produced by actually running the code; the Verified section quotes it in full. It builds on generics and TypeVar; read that first if T = TypeVar("T") is new.

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12 min read · 11 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

PythonTypeVarTuple in a generic Array, *Ts vs Unpack, typed *args and head/tail

Practice

PythonA typed call_soon(callback, *args) scheduler, and unpacking tuple types

Practice

Discussion

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